expand: Continue macro expansion after @dim conversion

This was recently introduced and was noticed because c:let/@name was not
being generated under certain circumstances.

* src/current/include/preproc/expand.xsl (preproc:expand): Continue applying
    templates after @dim expansion (rather than copying child nodes).

src/current/include/preproc/expand.xsl

@@ -157,7 +157,7 @@
       </xsl:choose>
     </xsl:attribute>
 
-    <xsl:sequence select="node()" />
+    <xsl:apply-templates select="node()" mode="preproc:expand" />
   </xsl:copy>
 </xsl:template>